Single-Shot Aspect Ratio and Orientation Imaging of Nanoparticles

Abstract

Plasmonic nanoparticles with surface plasmon resonance (SPR) and scattering response dependent on their geometry and surrounding environment are predestinated to be used as optical probes for sensing and imaging. Optical microscopy is capable of observing nanoparticles in various media, but their geometry remains hidden below the diffraction limit. Here, a wide-field optical imaging technique is demonstrated, restoring the aspect ratio and orientation of individual nanoparticles via the polarization anisotropy (PA) measurement of the scattered light. The PA is mapped into a single nanoparticle image, formed by decomposing the scattered light into longitudinal and transverse SPR modes and manipulating their angular momentum. The wide-field images provide the aspect ratio and orientation of many deposited nanoparticles allowing their assessment in heterogeneous suspensions or time-resolved measurements. In calibration experiments, orientation measurement accuracy and excellent sensitivity to nanoparticles with specific aspect ratios are demonstrated. Subsequently, the method is deployed in the automatic shape-dependent categorization of hundreds of nanoparticles in a heterogeneous mixture. The single-shot capability is demonstrated in the time-resolved imaging of the electrophoretic deposition process.